Thread Rating:
  • 2 Votes - 4 Average
  • 1
  • 2
  • 3
  • 4
  • 5
The colour of plugs when you take them out.
I have just taken my plugs out for cleaning because although the engine is running apparently ok there was difficulty in starting it on the handle. When all 4 plugs came out they were all dry with no signs of oiling but were jet black with carbon soot. In the old days when lead was in petrol plugs (if the engine was running ok) came out a greyish colour indicating that all was right as far as petrol/air mixture. Now we are on unleaded petrol what colour should you see if the engine is running ok.
The engine does now start on the handle since the plug cleaning.

John Mason
between a very light pink to light coffee colour
Most Haynes manuals have a page showing colours of plugs with an approximate diagnosis. I copied this and have it stuck on the workshop wall. It does tend to fade so I print another copy once a year or so.
Thank you both for your replies. This leads me to think that I maybe running on a to rich a mixture However adjustment of the mixture screw (the one on the top of the carb) only leads to uneven poor running which ever way you turn it. Adjusting to a position mid way between these two extreams of poor running does give smooth running without flat spots. Can anyone give any advice as to why it maybe running rich given the black sooty plugs. I should have said earlier my car is a 1935 Ruby with Zenith side draught carb.

John Mason
If you weaken the mixture, the car runs less well. If you richen the mixture the car runs less well. Perhaps that engine is trying hard to tell you something and that something is 'leave me alone'!! The object of the exercise is to get a car that runs well. The colour of the plug electrodes is of minor/no significance as long as that is achieved.

Hello John
I have a '37 Ruby and also suffered from the same problem as you.
However, when I asked on here I was advised to try running a slightly hotter plug to try and combat this.
I did this by changing from my usual NGK B6HS to a B5HS and the difference was immediate and very satisfying in that the plugs were a nice light grey colour.
The engine certainly feels a lot happier as well.
Got to be worth a try.
Hope it helps
The mixture screw only affects the tick over settings.
Its the jets that affect the actual mixture when running.
Its worth checking the float level and the needle valve and its seat.
The needle valve and seat will be worn after 70 plus years which means it will not seal as well as it did when new.
The Ruby head is well known for oiling its plugs. and as Graham has mentioned a change of heat range can help.
What's difference between   NGKb5hs and NGKbp5hs ?
The P stands for projected nose, with a Ruby head I feel they are a better option.
Location: Auckland NZ
I currently use Champion L86C in my high compression head and I was going to order a set of NGK B5HS (4210) as recommended by Graham Honnor.
So I emailed The Green Spark Plug Co. and told them I was using L86C and wanted a hotter plug to reduce oiling.
They replied with details of NGK B4H (4110) £2.15 whereas the NGK B5HS (4210) is £1.69.
So is there a ranges of hotter spark plugs as I also found NGK B6HS (4510) £1.38.
Also the NGK Copper Core Spark Plug BP5S (3011) is listed £1.79
I’m now confused as to which I should fit I as I don’t want to melt the pistons as I recently did on our 2015 Ford Kuga 1.5L Ecoboost with only 15000 on the clock but that’s another story, but I did get my money back from Ford.
I Think I’ll phone them tomorrow and try to establish the difference in plugs that dimensionally are all identical. Unless someone can answer this query Huh

Forum Jump:

Users browsing this thread: 1 Guest(s)